Defect modes, conformational disorder

The extensive studies of Snyder et al. (10) on the infrared spectra of hydrocarbons have also identified several CH2 wagging bands, which are nearly constant in frequency in the spectra of compounds with various chain lengths, that appear to be related to certain conformations, and are hence known as "defect modes. Changes in the relative intensity of these bands have been used to estimate the conformational disorder of the tails of micellar SDS (11), and the changes in packing of solid hydrocarbons as a function of temperature (12). 

Considerable experimental evidence (1-6,11) suggests that the methylene chains inside of a spherical micelle are almost as disordered as in the bulk liquid state (i.e. they contain a significant proportion of gauche conformers). The FTTR spectra of micellar SDS support this assertion, exhibiting CH2 stretching and scissoring band frequencies which are comparable to those found in the spectra of liquid hydrocarbons (1-6,11). A recent quantitative analysis of the CH2 defect modes of SDS has shown that the disorder of me methylene tails is similar to that found in liquid tridecane (11). 

Moreover, even in the case of regular constitution, configuration, and conformation, disorder may be present in the crystals because of the presence of defects in the mode of packing. Disorder in the packing may occur while some structural feature, for instance, some atoms or the axes of helical macromolecules, maintain periodic positions [137], The degree of disorder in the... 

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